Catalyst System for the Reduction of NOx and NH3 Emissions

ABSTRACT

This catalyst system simultaneously removes ammonia and enhances net NO x , conversion by placing an NH 3 —SCR catalyst formulation downstream of a lean NO x , trap. By doing so, the NH 3 —SCR catalyst adsorbs the ammonia from the upstream lean NO x , trap generated during the rich pulses. The stored ammonia then reacts with the NO x , emitted from the upstream lean NO x , trap—enhancing the net NO x , conversion rate significantly, while depleting the stored ammonia. By combining the lean NO x , trap with the NH 3 —SCR catalyst, the system allows for the reduction or elimination of NH 3  and NO x , slip, reduction in NO x . spikes and thus an improved net NO x , conversion during lean and rich operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/065,470, filed Oct. 22, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a catalyst system to facilitate thereduction of nitrogen oxides (NO_(x)) and ammonia from an exhaust gas.More particularly, the catalyst system of this invention includes a leanNO_(x) trap in combination with an ammonia selective catalytic reduction(NH₃—SCR) catalyst, which stores the ammonia formed in the lean NO_(x)trap during rich air/fuel operation and then reacts the stored ammoniawith nitrogen oxides to improve NO_(x) conversion to nitrogen when theengine is operated under lean air/fuel ratios. In an alternateembodiment, a three-way catalyst is designed to produce desirable NH₃emissions at stoichiometric conditions and thus reduce NO_(x) and NH₃emissions.

2. Background Art

Catalysts have long been used in the exhaust systems of automotivevehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides(NO_(x)) produced during engine operation into non-polluting gases suchas carbon dioxide, water and nitrogen. As a result of increasinglystringent fuel economy and emissions standards for car and truckapplications, it is preferable to operate an engine under leanconditions to improve vehicle fuel efficiency and lower CO₂ emissions.Lean conditions have air/fuel ratios greater than the stoichiometricratio (an air/fuel ratio of 4.6), typically air/fuel ratios greater than15. While lean operation improves fuel economy, operating under leanconditions increases the difficulty in treating some polluting gases,especially NO_(x).

Regarding NO_(x), reduction for diesel and lean burn gasoline engines inparticular, lean NO_(x), adsorber (trap) technologies have been widelyused to reduce exhaust gas NO_(x) emissions. Lean NO_(x) adsorbersoperate in a cyclic fashion of lean and rich durations. The lean NO_(x)trap functions by adsorbing NO_(x) when the engine is running under leanconditions—until the NO_(x) trap reaches the effective storagelimit—followed by NO_(x) reduction when the engine is running under richconditions. Alternatively, NO_(x) reduction can proceed by simplyinjecting into the exhaust a sufficient amount of reductant that isindependent of the engine operation. During this rich cycle, a shortrich pulse of reductants, carbon monoxide, hydrogen and hydrocarbonsreduces the NO_(x) adsorbed by the trap during the lean cycle. Thereduction caused during the rich cycle purges the lean NO_(x) adsorber,and the lean NO_(x) adsorber is then immediately available for the nextlean NO_(x) storage/rich NO_(x) reduction cycle. In general, poor NO_(x)reduction is observed if the air excess ratio λ is above 1. NO_(x)reduction generally increases over lean NO_(x) adsorbers as the λ ratiois decreased lower than 1. This air excess or lambda ratio is defined asthe actual air/fuel ratio divided by the stoichiometric air/fuel ratioof the fuel used. The use of lean NO_(x) adsorber (trap) technology, andin particular the rich pulse of reductants, can cause the λ ratio toreach well below 1.

Lean NO_(x), traps, however, often have the problem of low NO_(x),conversion; that is, a high percentage of the NO_(x), slips through thetrap as NO_(x). NO_(x), slip can occur either during the lean portion ofthe cycle or during the rich portion. The lean NO_(x), slip is oftencalled “VNO_(x) breakthrough”. It occurs during extended lean operationand is related to saturation of the NO_(x) trap capacity. The richNO_(x) slip is often called a “NO_(x) spike”. It occurs during the shortperiod in which the NO_(x) trap transitions from lean to rich and isrelated to the release of stored NO_(x) without reduction. Test resultsdepicted in FIG. 1 a have shown that during this lean-rich transition,NO_(x) spikes, the large peaks of unreacted NO_(x), accounts forapproximately 73% of the total NO_(x) emitted during the operation of alean NO_(x) trap. NO_(x) breakthrough accounts for the remaining 27% ofthe NO_(x) emitted.

An additional problem with lean NO_(x) traps arises as a result of thegeneration of ammonia by the lean NO_(x) trap. As depicted in FIG. 1 b,ammonia is emitted into the atmosphere during rich pulses of the leanNO_(x) adsorber. In laboratory reactor experiments, ammonia spikes ashigh as 600 ppm have been observed under typical lean NO_(x) adsorberoperation(see FIG. 1 b). While ammonia is currently not regulated,ammonia emissions are being closely monitored by the U.S. EnvironmentalProtection Agency; and, therefore, reduction efforts must be underway.Ammonia is created when hydrogen or hydrogen bound to hydrocarbonsreacts with NO_(x) over a precious metal, such as platinum. Thepotential for ammonia generation increases for a precious metal catalyst(such as a lean NO_(x) trap) as the λ ratio is decreased, as theduration of the rich pulse increases, and the temperature is decreased.There is thus an optimum lambda and rich pulse duration where themaximum NO_(x) reduction is observed without producing ammonia. Attemptsto enhance conversion of NO_(x) by decreasing the λ ratio of the richpulse duration leads to significant production of ammonia and thusresults in high gross NO_(x) conversion (NO_(x)→N₂→NH₃), but much lowernet NO_(x) conversion (NO_(x)→N₂).

In addition to nitrogen, a desirable non-polluting gas, and theundesirable NH₃ described above, N₂O is another NO_(x) reductionproducts. Like NH₃, N₂O is generated over NO_(x) adsorbers and emittedinto the atmosphere during rich pulses. The gross NO_(x) conversion isthe percent of NO_(x) that is reduced to N₂, N₂O and NH₃. The net NO_(x)conversion is the percent of NO_(x) that is reduced to nitrogen, N₂,only. Accordingly, the gross NO_(x) conversion is equal to the netNO_(x) conversion if nitrogen is the only reaction product. However, thenet NO_(x) conversion is almost always lower than the gross NO_(x)conversion. Accordingly, a high gross NO_(x) conversion does notcompletely correlate with the high portion of NO_(x) that is reduced tonitrogen.

The NO_(x) conversion problem is magnified for diesel vehicles, whichrequire more than a 90% NO_(x) conversion rate under the 2007 U.S. TierII BIN 5 emissions standards at temperatures as low as 200° C. Whilehigh NO_(x) activity is possible at 200° C., it requires extrememeasures such as shortening the lean time, lengthening the rich purgetime, and invoking very rich air/fuel ratios. All three of thesemeasures, however, result in the increased formation of NH₃ or ammonia.Accordingly, while it may be possible to achieve 90+% gross NO_(x)conversion at 200° C., to date there has not been a viable solution toachieve 90+% net NO_(x) conversion.

Accordingly, a need exists for a catalyst system that eliminates NO_(x)breakthrough during the lean operation as well has the NO_(x) spikesduring the lean-rich transition period. There is also a need for acatalyst system that is capable of improving net NO_(x) conversion.Finally, there is a need for a catalyst system capable of reducingammonia emissions.

SUMMARY OF THE INVENTION

This invention provides a solution for all of the above problems and, inparticular, reduces or eliminates ammonia emissions and improves the netNO_(x) conversion of the catalyst system. These problems are solved bysimultaneously removing ammonia and enhancing NO_(x) conversion with theuse of an NH₃—SCR catalyst placed downstream of the lean NO_(x) adsorbercatalyst, as shown in FIG. 2. The NH₃—SCR catalyst system serves toadsorb the ammonia emissions from the upstream lean NO_(x) adsorbercatalyst generated during the rich pulses. Accordingly, as shown in FIG.2, the ammonia emissions produced by the lean NO_(x) adsorber is storedand effectively controlled by the NH₃—SCR catalyst rather than beingemitted. This reservoir of adsorbed ammonia then reacts directly withthe NO_(x) emitted from the upstream lean NO_(x) adsorber. As a result,as shown in FIG. 3, the overall net NO_(x) conversion is enhanced from55% to 80%, while depleting the stored ammonia, as a function of the SCRreaction: NH₃+NO_(x)→N₂. The NH₃—SCR catalyst is then replenished withammonia by subsequent rich pulses over the lean NO_(x) adsorber.

During the lean cycle for this lean NO_(x) adsorber+NH₃—SCR system, theNO_(x) breakthrough from the upstream lean NO_(x) adsorber is reducedcontinuously as it passes over the NH₃—SCR until the reservoir ofammonia is depleted. In addition, during the rich cycle, large spikes ofunreacted NO_(x), are created. The downstream NH₃—SCR catalyst thusserves to dampen these large NO_(x), spikes by reacting the unreactedNO_(x), with the reservoir of stored ammonia emitted from the leanNO_(x), adsorber. In general, the combination of the lean NO_(x),adsorber +NH₃—SCR catalyst system allows for the reduction, orelimination, of ammonia emissions and NO_(x), slip, i.e., reduction ofNO_(x), breakthrough and NO_(x), spikes and, therefore, improved netNO_(x), conversion during lean and rich operation.

Additionally, under this invention, urea and/or ammonia does not need tobe injected into the exhaust system to effectuate the reaction betweenNO_(x) and ammonia. Rather, the ammonia is automatically generated fromthe NO_(x) present in the exhaust gas as it passes over the preciousmetal lean NO_(x) adsorber during the rich pulses. The generated ammoniais then stored on the downstream NH₃—SCR catalyst, to react with theunreacted NO_(x), and thereby convert the unreacted NO_(x) to nitrogen.

The NH₃—SCR catalyst thus serves to adsorb the ammonia from the upstreamlean NO_(x) adsorber catalyst generated during the rich pulses. Underthis system, the ammonia is stored and effectively controlled ratherthan being emitted. This reservoir of adsorbed ammonia then reactsdirectly with any NO_(x) emitted from the upstream lean NO_(x) adsorber.As a result, the overall net NO_(x) conversion is enhanced from 55% to80%, while the overall gross NO_(x) conversion is enhanced from 68% to82%, as shown in FIG. 3.

In one alternative embodiment of this invention, the catalyst system canbe optimized and NO_(x) reduction increased by vertically slicing thelean NO_(x) trap and NH₃—SCR catalyst substrates to create separatecatalyst zones, such that the catalytic converter shell or can wouldhave alternating sections of lean NO_(x), trap and NH₃—SCR catalysts, asshown in FIGS. 4 a, 4 b and 4 c. Under this embodiment, bothtechnologies, the lean NO_(x), trap formulation and the NH₃—SCRformulation, can be incorporated into a single substrate and/or a singleconverter can rather than placing the NH₃—SCR catalyst downstream of thelean NO_(x), adsorber as two separate and distinct catalyst substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a graph illustrating the NO_(x), spikes that occur duringthe NO_(x), trap lean-rich transition;

FIG. 1 b is a graph illustrating NO_(x), and NH₃ emissions from atypical prior art lean NO_(x), adsorber system;

FIG. 2 depicts the lean NO_(x), and NH₃—SCR catalyst system of thepresent invention;

FIG. 3 depicts reduced NO_(x), emissions and NH₃ emissions as a resultof the use of the lean NO_(x), and NH₃—SCR catalyst system of thepresent invention, as shown in FIG. 2;

FIGS. 4 a, 4 b, and 4 c depict three different zoned catalystembodiments of the lean NO_(x), and NH₃—SCR catalyst system;

FIGS. 5 a, 5 b, and 5 c provide graphs illustrating the reduced levelsof NO_(x), and NH₃ emissions resulting from each of the three zonedcatalyst embodiments depicted in

FIGS. 4 a, 4 b, and 4 c at a 250° C. inlet gas temperature and operatingat a 50 second lean cycle and 5 second rich cycle;

FIGS. 6 a, 6 b and 6 c provide graphs illustrating the reduced levels ofNO_(x), and NH₃ emissions resulting from each of the three zonedcatalyst embodiments depicted in FIGS. 4 a, 4 b and 4 c at a 200° C.inlet gas temperature and operating at a 25 second lean cycle and a 5second rich cycle;

FIGS. 7 a, 7 b and 7 c show three proposed examples of washcoatconfigurations incorporating the lean NO_(x) trap and NH₃—SCRformulations into the same substrate;

FIG. 8 is a graph illustrating the impact of NO_(x) conversion afterhydrothermal aging; and

FIG. 9 depicts a modified three-way catalyst and NH₃—SCR catalyst systemof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In this invention, net NO_(x) conversion is improved and ammoniaemissions reduced through the use of a lean NO_(x) trap and NH₃—SCRcatalyst system which operate together to produce and store ammonia andreduce NO_(x) to nitrogen. In so doing, the catalyst system of thepresent invention solves three problems of lean NO_(x) traps; namely,reducing NO_(x) breakthrough, NO_(x) spikes and ammonia emissions.

In order to meet increasingly stringent fuel economy standards, it ispreferable to operate an automotive engine under lean conditions.However, while there is improvement in fuel economy, operating underlean conditions has increased the difficulty in reducing NO_(x)emissions. As an example, for a traditional three-way catalyst, if theair/fuel ratio is lean even by a small amount, NO_(x) conversion dropsto low levels. With traditional three-way catalysts, the air/fuel ratiomust be controlled carefully at stoichiometric conditions to maximizereduction of hydrocarbons, carbon monoxide and NO_(x).

Throughout this specification, NO_(x) refers to nitrogen oxides, whichinclude nitrogen monoxide NO and nitrogen dioxide NO₂. Further, leanNO_(x) adsorber and lean NO_(x) trap are used interchangeably throughoutthis specification.

To achieve NO_(x) reduction, under lean operating conditions, one optionis the inclusion of a lean NO_(x) trap. While the lean NO_(x) trap isgenerally effective in NO_(x) reduction, lean NO_(x) traps are known tohave the problems referred to as “NO_(x) slip” which includesbreakthrough of NO_(x) during the extended lean operation of the NO_(x)trap and also NO_(x) spikes generated during the transition from thelean to the rich cycle.

NO_(x) spikes, or NO_(x) emissions during the lean-rich transition, arebelieved to occur due to the exothermic heat generated from theoxidation of reductants, carbon monoxide, hydrocarbons and hydrogen, bythe oxygen released from the oxygen storage material—the temperaturerise can be as high as 80-100° C.

The problem of NO_(x) spikes is illustrated in FIG. 1 a, and the problemof insufficient net NO_(x) conversion is illustrated in FIG. 1 b. FIG. 1b depicts laboratory reactor data of a lean NO_(x) adsorber systemoperating in an 85 second lean and 5 second rich cyclic pattern. Theplot in FIG. 1 b shows the nitrogen species concentration as a functionof time. The laboratory reactor data depicted in FIG. 1 b resulted froma catalyst having an engine swept volume (ESV) of 100%. Additionally,the reactor used to obtain the results in FIG. 1 b was at a temperatureof 300° C. To begin the cycle, 500 ppm of nitrogen oxide was fed intothe reactor where much of it was stored during the 85 second leanduration. During the 5 second rich duration, nitrogen oxide was reduced;however, a significant amount of ammonia was formed. As illustrated inFIG. 1 b, the data shows ammonia spikes as high as 600 ppm under typicallean NO_(x). adsorber operation. Conversion, however, is generallyimproved as the λ ratio is decreased during the rich pulse. Decreasingthe λ ratio also leads to significant production of ammonia and thusresults in high gross NO_(x), conversion (NO_(x)→N₂+NH₃), but much lowernet NO_(x), conversion (NO_(x), N₂) . As illustrated in FIG. 1 b, thenet NO_(x), conversion to nitrogen for this lean NO_(x), adsorber systemwas only 55%.

Under the catalyst system of this invention, ammonia is reduced and thenet NO_(x), conversion improved simultaneously by placing an NH₃—SCRcatalyst formulation downstream of the lean NO_(x), adsorber catalyst,as shown in FIG. 2.

FIG. 2 is an illustration of the catalyst system of this invention,which is capable of simultaneously eliminating ammonia emissions andimproving net NO_(x), conversion. As illustrated in FIG. 2, NO_(x),produced during engine operation is stored by the lean NO_(x), adsorberduring the lean cycle. Following the lean cycle, during the rich cycleof the lean NO_(x), adsorber, NO_(x), is reduced and ammonia generated.The lean NO_(x), adsorber stores much of the NO_(x), during the leanoperation and then reduces NO_(x), during rich pulses of the reductants.During the same rich pulses, significant amounts of ammonia aregenerated, as further illustrated in FIG. 1. As illustrated in FIG. 2,the lean NO_(x), adsorber emits NO, NO₂, NH₃, and N₂O. These same gasesthen pass through the NH₃—SCR, where NH₃ is stored. Accordingly, theaddition of the NH₃—SCR catalyst downstream allows for the adsorption ofNH₃ and subsequent reaction with any NO_(x), that slips through theupstream lean NO_(x), adsorber, which thus improves the overall netNO_(x), conversion (NH₃+NO→N₂). As can be seen in FIG. 2, the catalystsystem of this invention results in a significant net NO_(x), conversionimprovement, the elimination of ammonia emissions, and the production ofnon-polluting gases nitrogen and N₂O.

It should be noted that for diesel applications, lean NO_(x), adsorbersmust operate at lower temperatures compared to gasoline lean NO_(x),adsorbers since the exhaust temperatures of diesel engines aresignificantly lower. More ammonia is generated at 200° C. than at 300°C. over lean NO_(x) adsorbers, and thus the catalyst system of thisinvention has an even greater potential for diesel applications.Likewise, the problem of NO_(x) spikes is more critical at highertemperatures, the temperatures used for gasoline applications; and thusthe catalyst system of this invention is beneficial to control theunreacted NO_(x) spikes that result from the operation of a lean NO_(x)adsorber at operating temperatures typical for gasoline lean NO_(x)adsorber applications.

The NH₃—SCR catalyst thus serves to adsorb the ammonia producednaturally from the upstream lean NO_(x) adsorber catalyst generatedduring the rich pulses. As a result, the NH₃—SCR catalyst stores theammonia, controlling it rather than allowing it to be emitted into theatmosphere. This reservoir of adsorbed NH₃ in the NH₃—SCR catalystreacts directly with the NO_(x) emitted from the upstream lean NO_(x)adsorber (trap).

In general, this invention works to clean NO_(x) emissions—and thus hasapplicability for stationary sources as well as for moving vehicles.This invention may be used to reduce NO_(x) emissions for nitric acidplants, or any other stationary source that requires the reduction ofNO_(x) emissions. This invention is nonetheless particularly directedfor use with gasoline and diesel vehicles which, unlike stationarysources, have a wide range of operating parameters, especiallytemperature parameters—which cannot be precisely controlled. The presentinvention has the ability to store large quantities of ammonia across abroad temperature range to effectuate the reaction between ammonia andnitrogen oxides and thereby convert NO_(x) to nitrogen.

As illustrated in FIG. 3, laboratory experiments have demonstrated thatthe use of a lean NO_(x) adsorber plus NH₃—SCR catalyst system improvesnet NO_(x) conversion from 55%, as illustrated in FIG. 1, to 80%. FIG. 3is a graph displaying laboratory data obtained using the catalyst systemof this invention, wherein NO_(x) ppm are charted as a function of time.As illustrated in FIG. 3, the catalyst system of this inventioncompletely eliminated the ammonia spikes created during the rich pulsesof the lean NO_(x) adsorber. In this system, ammonia is stored on theNH₃—SCR catalyst where it reacts with NO_(x) during the 85 second leanduration, which thus improves the net NO_(x) conversion from 55% to 80%with no additional fuel economy penalty. As shown in FIG. 3, theimproved net NO_(x) conversion can be observed by the much narrowerprofile—zero ppm NO_(x) is emitted for a significant amount of time ascompared to the graph shown in FIG. 1 of a system lacking the NH₃—SCR+lean NO_(x) adsorber combination.

The reaction between the stored ammonia and NO_(x)increases the overallnet NO_(x) conversion, which is enhanced from 55%—the amount of NO_(x)converted in prior art lean NO_(x) trap systems—to 80%—as a result ofthe combination of a lean NO_(x) trap and NH₃—SCR catalyst system.Moreover, in addition to improving net NO_(x) conversion, the ammoniastored in the NH₃—SCR catalyst is depleted during the SCR reactionwherein ammonia and nitrogen oxide are reacted to produce nitrogen. TheNH₃—SCR catalyst is replenished with ammonia by subsequent rich pulsesover the lean NO_(x) adsorber that causes a portion of the NO_(x), toreact with hydrogen to form ammonia.

It should be noted that no urea or ammonia needs to be injected into theexhaust system to effectuate the reaction between ammonia and NO_(x).Rather, the ammonia is naturally generated from the NO_(x) present inthe exhaust gas as it passes over the lean NO_(x) trap during richpulses. More specifically, ammonia is naturally created during the fuelrich cycle of the lean NO_(x) trap. Ammonia is naturally produced as itpasses over the precious metal active component of the lean NO_(x) trap.Similarly, the ammonia could also be generated in a conventionalprecious metal based TWC located upstream of a LNT/NH₃—SCR system.

For this invention, the lean NO_(x) trap is optimized for ammoniageneration by removing oxygen storage capacity (OSC) and therebyenhancing the rich cycle, and thus creating a greater quantity ofammonia for reaction with the NO_(x) in the downstream NH₃—SCR catalyst.In a preferred embodiment, the lean NO_(x) trap includes platinum as theprecious metal. Platinum is the preferred precious metal because it isbelieved that a greater quantity of NH₃ is produced over platinum thanrhodium, palladium and/or a combination of the precious metals.Nonetheless, other precious metals such as palladium and rhodium, andthe combination of one or more of the precious metals platinum,palladium and rhodium may also be used to generate NH₃.

Additionally, the lean NO_(x) trap of this invention preferably includesa “VNO_(x) adsorbing material” or NO_(x) storage component/material,which can be alkali and alkali earth metals such as barium, cesium,and/or rare earth metals such as cerium and/or a composite of cerium andzirconium. Although an alternative catalyst formulation that does notcontain a NO_(x) storage component but generates ammonia from NO_(x),may also be utilized, in the most preferred embodiment, the NO_(x),storage material should have the ability to store NO_(x), at lowtemperature ranges, specifically in the range of 150° C.-300° C. The NH₃thermodynamic equilibrium under rich conditions is maximized during thetemperature range of 150° C.-300° C.

In general, to increase the NO_(x) storage function of the lean NO_(x)trap and effectuate the NO_(x) conversion reaction, in the preferredembodiment, the lean NO_(x) trap has the following characteristics: (1)the inclusion of platinum as the precious metal; (2) the ability tostore NO_(x) between 150° C. and 500° C. during the lean portion of thecycle; (3) the ability to maximize the duration of the lean NO_(x) traprich cycle; (4) the ability to generate ammonia at the 150° C.-500° C.temperature range; (5) minimize OSC to lessen fuel penalty; and (6)lower lambda to generate more ammonia. Ammonia production is maximizedat the preferred temperature range, 150° C.-300° C.—which alsocorrelates with the steady state equilibrium range for ammonia creation.It bears emphasis that other NO_(x) storage components may be utilized,especially for stationary sources, where sulfur poisoning does not posea threat.

Most simply, the NH₃—SCR catalyst may consist of any material orcombination of materials that can adsorb ammonia and facilitate theNO_(x)+NH₃ to yield nitrogen. The NH₃—SCR catalyst should preferably bemade of a base metal catalyst on a high surface area support such asalumina, silica, titania, zeolite or a combination of these. Morepreferably, the NH₃—SCR catalyst should be made of a base metal selectedfrom the group consisting of Cu, Fe and Ce and/or a combination of thesemetals, although other base metals may be used. Base metals generallyare able to effectuate NO_(x), conversion using ammonia while both thebase metals and the high surface support material serves to store NH₃.The base metal and high surface area support such as zeolite selectedshould preferably be one that can store NH₃ over the widest possibletemperature range. Likewise, the base metal selected is preferably onethat can convert NO and NO₂ to N₂ across the widest possible temperaturerange and the widest range of NO/NO₂ ratios.

The advantage of the catalyst system of this invention is the use of acombination of a lean NO_(x), trap and an NH₃—SCR catalyst. The use of alean NO_(x), trap in the present system allows for much greater storageof NO_(x)R, because the NO_(x), breakthrough that would otherwise happencan be controlled by the NH₃—SCR catalyst. Additionally, the use of alean NO_(x), trap as part of this system allows for the operation of theengine at lean conditions for a longer time, and thus provides improvedfuel economy. If, for example, a three-way catalyst is used as theNO_(x) storage mechanism, NO_(x) storage is significantly limited, aswell as the production of ammonia. To maximize the reduction ofemissions, a three-way catalyst must be operated at stoichiometricconditions. Accordingly, unless the three-way catalyst is run on therich side 100% of the time, ammonia production is significantly lessthan for a typical lean NO_(x) trap. As set forth above, the efficiencyof a three-way catalyst is compromised if it is operated at conditionsother than at stoichiometric conditions. Thus the combination of a leanNO_(x) trap and NH₃—SCR catalyst allows for significant NO_(x) storageand ammonia production and thus increases net NO_(x) conversion.

In a preferred embodiment, the lean NO_(x) trap and NH₃—SCR catalystconstitute alternating zones in a single substrate and/or a singlecatalytic converter can. This zoned design, as shown in three differentembodiments in FIGS. 4 a-4 c, is believed to maximize the reactionbetween ammonia and NO_(x).

As illustrated in FIG. 4, three zoned catalyst system embodiments wereevaluated on a laboratory flow reactor. The total catalyst systemdimensions were held constant at a 1″ diameter and 2″ length. The firstsystem, labeled “4 a”, had a 1″ long lean NO_(x) trap followed by a 1″long NH₃—SCR catalyst. In the second system, labeled “4 b”, the catalystsamples were sliced in half to yield alternating ½″ long sections.Finally, in the third system, labeled “4 c”, the same catalyst sampleswere further cut in half to yield ¼″ long sections, again of the leanNO_(x) trap and NH₃—SCR catalyst technologies. It should be noted thateach time the catalysts were sliced, as shown in “4 b” and “4 c”, theoverall length of the catalyst system was reduced slightly,approximately 3/16″ total. The alternating lean NO_(x) trap and NH₃—SCRcatalyst zones can be created in a single substrate or the lean NO_(x)trap and NH₃—SCR catalyst prepared, cut as desired and then placedadjacent one another in a single can. The zones are preferably formed ina single substrate. However, cut substrates placed in alternatingfashion also exhibit improved net NO_(x) conversion.

Under the zoned catalyst designs shown in FIGS. 4 a-4 c, wherealternating lean NO_(x) and NH₃—SCR catalyst zones are provided, theammonia formed by the lean NO_(x) trap is believed to be immediatelyadsorbed by the NH₃—SCR catalyst for use in the NO_(x) conversionreaction. It is further believed that the greater the separation betweenthe lean NO_(x) trap and the NH₃—SCR catalyst, the greater chance thereis for the ammonia to be converted back into NO_(x). It is furtherbelieved that oxygen is more abundant in the back of a catalystsubstrate and thus the oxygen may be available to effectuate theunwanted conversion of the ammonia back to nitrogen oxide. Accordingly,if the catalyst substrate is too long, there may be some undesiredconversion that takes place; and thus in a preferred embodiment, thesubstrate is designed so that ammonia is available for immediatereaction with NO_(x).

FIGS. 5 a-5 c illustrate laboratory reactor data of the three differentzoned catalyst system embodiments shown in FIGS. 4 a-4 c. Thislaboratory data was obtained with the three catalyst systems operatingat a 250° C. inlet gas temperature and operating with 50 second lean and5 second rich cycles. Additionally, the inlet concentration of theNO_(x) feed gas was 500 ppm and the overall space velocity was 15,000per hour. As illustrated in FIGS. 5 a-5 c, with the use of a two-zonedcatalyst system as depicted in FIG. 5 a, approximately 50 ppm of NO isemitted. This two-zone catalyst system resulted in a gross NO_(x)conversion of 95% and a net NO_(x) conversion of 66%. The four-zonecatalyst embodiment, depicted as FIG. 5 b, significantly reduced NO_(x)emissions, well below the 15 ppm range, to result in gross NO_(x)conversion of 99% and a net NO_(x) conversion of 86%. Finally, asillustrated by the eight zoned catalyst embodiment, FIG. 5 c, grossNO_(x) conversion is 100% and net NO_(x) conversion is 97.5%. Theimprovement comes from the reduction of N₂O, elimination of the NH₃breakthrough and reduction of NO_(x). Accordingly, as the catalystsystem is zoned down from 1″ sections to ¼″ sections, the test resultsrevealed an associated improvement in net NO_(x) conversion.

As shown in FIGS. 5 a-5 c, a zoned catalyst, with alternating leanNO_(x) and NH₃—SCR catalysts in 1″ to ¼″ sections significantly improvesthe net NO_(x) conversion from 66% to 97.5%. In addition, the grossNO_(x) conversion is improved from 95% to 100%. In general, theimprovement in the net NO_(x), conversion is the function of theelimination of the ammonia slip, reduction in N₂O, and extra NO_(x)reduction related to the NH₃ +NO_(x) reaction on the NH₃—SCR catalyst.It is further believed that the drop in N₂O emissions is likely due to ahigher fraction of the NO_(x) reduction reaction proceeding on theNH₃—SCR catalyst rather than the lean NO_(x) trap. NO_(x) reduction overa platinum-containing-lean NO_(x) trap results in high levels of N₂Ogeneration, whereas the NH₃—SCR catalyst has a high selectivity tonitrogen.

FIGS. 6 a-6 c depicts laboratory data obtained using the three-zonedcatalyst embodiments originally shown in FIGS. 4 a-4 c at a 200° C.inlet gas temperature operating with a 25 second lean cycle and a 5second rich cycle. As compared to FIGS. 5 a-5 c, it should be noted thatshortening the lean time from 50 seconds, as used in FIGS. 5 a-5 c, to25 seconds, resulted in a substantial higher steady emission ofammonia—a fact which results in reduced net NO_(x) conversion rates, ascompared to the data charted in FIGS. 5 a-5 c. As can be seen in FIGS. 6a-6 c, the use of smaller zoned sections from two zones to eight zones,and thus 1″ sections down to ¼″ sections, as illustrated in FIGS. 6 aand 6 c, improves the net NO_(x) conversion from 50% to 81%. Again, thisimprovement is believed to come mainly from the reduction of ammoniabreakthrough and a small reduction in N₂O emissions. This lab data wasobtained with an inlet concentration of the NO_(x) feed gas at 500 ppmand an overall space velocity at 15,000 per hour.

As set forth above, in the preferred embodiment, the lean NO_(x) trapwashcoat and NH₃—SCR washcoat are combined in a single substrate ratherthan placing the NH₃—SCR formulation downstream of the lean NO_(x)adsorber as two separate catalyst substrates. Under this embodiment, thecatalyst formulations can be incorporated together by mixing or layeringthe washcoats on a substrate.

FIGS. 7 a-7 c show three proposed washcoat configurations incorporatingthe lean NO_(x) trap and NH₃—SCR formulations into the same substrate.As shown in FIGS. 7 a and 7 b, the first and second proposedconfigurations have the lean NO_(x) trap and NH₃—SCR washcoatformulations on the bottom and top layer, respectively. It is believedthat the top layer could be a highly porous structure that allows betterand faster contact between the chemicals and gas phase and the activesites in the second layer. The third configuration, as shown in FIG. 7c, involves the use of a one layer washcoat containing both lean NO_(x)trap and NH₃—SCR washcoat formulations. Under this third configuration,shown in FIG. 7 c, the washcoat composition of the lean NO_(x) trap andNH₃—SCR catalyst could be homogeneously or heterogeneously mixed. For aheterogeneously mixed composition, the formulation of the lean NO_(x)trap and NH₃—SCR catalyst are separated. However, they contact eachother in varying degrees by controlling the size of the grainstructures. The homogeneously mixed composition allows for a moreintimate contact between the two formulations and is thus preferred.

The invention also contemplates engineering such combinations within thepores of the monolithic substrate. An example of this is incorporatingwashcoat into porous substrates used for filtering diesel particulatematter. Thus, this lean NO_(x) trap/NH₃—SCR catalyst concept can beintegrated into diesel particulate matter devices.

This very active SCR reaction of NO_(x) and ammonia proceeds with orwithout oxygen present. Koebel et al. reports that the fastest SCRreaction involves equal molar amounts of NO and NO₂. NO and NO₂ thenreact with two NH₃ to yield N₂ in the absence of oxygen. In contrast,the lean NO_(x) adsorber reaction of NO_(x) plus CO is highly reactiveonly in an oxygen-free environment. In a lean NO_(x) adsorber system,NO_(x) is adsorbed during the lean cycle duration, NO_(x) is notreduced. Accordingly, NO_(x) reduction is limited to only the rich pulsetime duration. On the other hand, the lean NO_(x) adsorber+NH₃—SCRcatalyst system allows for NO_(x) reduction reaction to proceed duringboth the lean and rich time durations. Accordingly, ammonia as areductant can be considered as a much more robust reductant than carbonmonoxide.

As set forth above, the fastest SCR reaction involves equal molaramounts of NO and NO₂. Accordingly, FIG. 8 illustrates the impact ofvarying NO:NO₂ ratios after hydrothermal aging. FIG. 8 is a graph ofthree NH₃—SCR catalyst formulations over a wide NO:NO₂ range. In thelaboratory, it was possible to control the NO:NO₂ ratio entering thedownstream NH₃—SCR catalyst. Accordingly, the NO:NO₂ ratio entering theNH₃—SCR catalyst was solely dependent on the upstream lean NO_(x)adsorber. In some cases, the majority of the feed NO_(x) (especiallyNO_(x) spikes) are made up of mostly NO rather than NO₂. Accordingly, itis believed that the catalyst formulations of this invention willenhance reported net NO_(x) efficiency—and thus the preferred catalystis one that is capable of operating across the broadest range of NO:NO₂ratios, and at a full spectrum of temperature ranges.

In general, since NH₃—SCR catalysts do not contain precious metals, theyare significantly less costly than a typical lean NO_(x) trap.Accordingly, it is more cost effective to have an overall catalystsystem containing a lean NO_(x) trap adsorber and an NH₃—SCR catalystsystem, rather than one that uses two lean NO_(x) trap adsorbers.Additionally, the incorporation of both a lean NO_(x), trap and NH₃—SCRwashcoat into a single substrate will significantly reduce substratecosts.

In another embodiment of this invention, NH₃ and NO_(x), in an exhauststream are reduced using a stoichiometric three-way catalyst system.This three-way catalyst system has particular application for highspeed/high flow rate conditions (i.e., US06 conditions). Currently,three three-way catalysts are used for such high speed conditionapplications, wherein the third three-way catalyst is primarily directedto NO_(x) removal for high speed/high flow rate conditions. Under thisalternate embodiment, the third three-way catalyst can be substitutedwith an NH₃—SCR catalyst to store NH₃ for reaction with NO_(x) toimprove net NO_(x) conversion, eliminate NH₃ emissions and reducecatalyst costs.

To improve net NO_(x) and NH₃ reduction, the second three-way catalystcan be modified to enhance the three-way catalyst's ability to generateNH₃ emissions. To this end, in a preferred embodiment, the three-waycatalyst is designed to generate desirable NH₃ creation by usingplatinum as the precious metal of the three-way catalyst, by placingplatinum on the outer layer of the three-way catalyst to maximize theNO+H₂—NH₃ reaction. Likewise, the oxygen storage capacity (OSC) of thethree-way catalyst can be removed to further promote the creation of“desirable” NH₃. By doing so, the NH₃ purposely generated during richoperation can then be stored by the NH₃—SCR catalyst for subsequentreaction with NO_(x) emissions, and thereby control both NO_(x) and NH₃emissions under all operating conditions.

When a car is operated under rich conditions, the air/fuel ratio is lessthan 14.6, hydrogen is produced in the exhaust via the water-gas shiftreaction: CO+H₂O→CO₂+H₂. The hydrogen that is produced then reacts withNO_(x), as it passes over the precious metal surface to create“desirable” ammonia. The ammonia produced is then stored on an NH₃—SCRcatalyst to help reduce net NO_(x), conversion. The reaction ofNO_(x)+NH₃—N₂+H₂O can then take place on a separate NH₃ selectivecatalyst, capable of converting NO₂ and NO to N₂.

As shown in FIG. 9, a stoichiometric three-way catalyst/NH₃—SCR catalystsystem 10 is depicted, including a first three-way catalyst 14positioned in close proximity to the engine 12 to reduce cold startemissions. The second three-way catalyst 16 is modified as describedabove to enhance the ability of the second three-way catalyst 16 togenerate NH₃ emissions. Downstream of the second three-way catalyst 16is an NH₃—SCR catalyst 18 that functions to store NH₃ produced by themodified second three-way catalyst 16 for reaction with NO_(x)emissions, to reduce both NO_(x) and NH₃ emissions.

By substituting the third three-way catalyst as currently used with anNH₃—SCR catalyst and thereby eliminating the need for a third preciousmetal containing catalyst, significant cost savings can be achieved.

It should further be noted that this invention also contemplates the useof a three-way catalyst, in combination with a lean NO_(x) trap and anNH₃—SCR catalyst.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

1. A method of purifying exhaust gas of an internal combustion engine, the method comprising: generating ammonia in an exhaust system of said engine by a reaction of NO_(x) and reducing components in exhaust gases when the exhaust gases are in a reducing state; and removing NO_(x) downstream of the generated ammonia by adsorbing NO_(x) in the exhaust gases when the exhaust gases are in an oxidizing state, and reducing the adsorbed NO_(x) to generate ammonia and retaining the generated ammonia when the exhaust gases are in the reducing state.
 2. An exhaust gas purifying apparatus for an internal combustion engine, comprising: ammonia generating device, provided in an exhaust system of said engine, for generating ammonia by a reaction of NO_(x) and reducing components in exhaust gases when the exhaust gases are in a reducing state; and NO_(x) removing device, provided downstream of said ammonia-generating device, for adsorbing NO_(x) in the exhaust gases when the exhaust gases are in an oxidizing state, said NO_(x) removing device reducing the adsorbed NO_(x) to generate ammonia and retaining the generated ammonia when the exhaust gases are in the reducing state. 